SELECT * 
FROM ar_customers

customer#	customer_name	customer_address	customer_province	customer_rating
1	bill	123 main st.	ab	1
2	ed	456 main st.	bc	2
3	john	789 main st.	ab	3

SELECT * 
FROM ar_invoices

invoice#	customer#	salesperson#	invoice_date	invoice_amount	amount_paid	current_month_interest_charges	total_interest_charges
1	1	1	2019-06-17 10:43:53.727	100.00	1.00	0.00	0.00
2	2	2	2019-05-28 10:43:53.727	50.00	0.00	5.00	10.00
3	3	2	2019-06-17 10:43:53.727	70.00	0.00	10.00	20.00
4	2	1	2019-07-07 10:43:53.727	80.00	0.00	0.00	0.00

SELECT * 
FROM ar_salespeople

salesperson#	salesperson_name
1	bob
2	mary
3	tim

SELECT salesperson_name,
	MAX([Total Sales]) AS [Highest Earner]
FROM
	(SELECT salesperson_name, 
		   SUM(invoice_amount) AS [Total Sales]
	FROM ar_invoices
	INNER JOIN ar_salespeople
		ON ar_invoices.salesperson# = ar_salespeople.salesperson#
	GROUP BY salesperson_name) x
GROUP BY salesperson_name

salesperson_name	Highest Earner
bob	180.00
mary	120.00